Noise that can be heard inside a vehicle is generated due to many factors, such as engine operation, rough road conditions, vibrations from the suspension system and wind noise. To reduce such noise, there are generally two methods: 1) a passive noise control method to reduce noise by installing sound absorbing material inside the vehicle, and 2) an active noise control (ANC) method to reduce noise by canceling out the noise by outputting a control signal having an opposite phase to the noise. As today's consumers are demanding more comfortable and more quiet cars, the ANC method is growing popular because it produces remarkable effects in damping the noise like vehicle engine noise, as compared to the passive noise control method.
In general, an ANC system employed in vehicles consists of a microphone, a sensor for detecting noise, a digital signal processor (DSP) module comprised of e.g., an adaptive digital filter and a signal controller for reversing the phase of the detected noise and a speaker for reproducing amplified noise. The principle of how the ANC system works is damping or canceling a sound pressure level of noise by generating an artificial sound that has the same amplitude but opposite phase to the noise inside the vehicle and overlapping the artificial sound and the noise.
To make the ANC system efficient, a noise reducing signal having an opposite phase to the phase of the noise in the vehicle should be generated.
The noise reducing signal is generated based on a vibration signal generated from the suspension system, in some embodiments. Accordingly, efficient noise reduction is achieved only by generating the noise reducing signal based on a vibration signal measured from a position at which the vibration signal highly contributes to the noise in the interior of the vehicle. Since the vibration signal is generated from the suspension system, a sensor (or sensors) is commonly installed around the suspension system normally located on the exterior of the vehicle. This may cause an increase in vehicle production expense and sensor maintenance expense due to the need for a package, or an apparatus, to protect the sensor against external environments and aging.